Principle and application of electret microphone
There are two ways to connect the electret microphone to the circuit:
Source output and drain output. The source output is similar to the emitter output of a crystal transistor. Three lead wires are required. Drain D is connected to the positive pole of the power supply. A resistor Rs is connected between the source S and the ground to provide the source voltage, and the signal is output from the source through the capacitor C. The braided wire is grounded for shielding. The output impedance of the source output is less than 2k, the circuit is relatively stable, and the dynamic range is large. But the output signal is smaller than the drain output. The drain output is similar to the common emitter of the transistor. Only two leads are required. A drain resistor RD is indirectly connected between the drain D and the positive electrode of the power supply, and the signal is output from the drain D through the capacitor C. The source electrode S is grounded together with the braided wire. The drain output has voltage gain, so the microphone sensitivity is higher than the source output, but the circuit dynamic range is slightly smaller.
The size of Rs and RD should be determined according to the power supply voltage. Generally, it can be selected from 2.2 to 5.1k. For example, when the power supply voltage is 6V, Rs is 4.7k, and RD is 2.2k. In the output circuit of Fig. 3, if the power supply is positively grounded, you only need to swap D and S, and it can still become the source and drain output. A voice control circuit preamplifier stage of the two different connections of the source output and the drain output of the electret microphone. Finally, let me explain that no matter the source output or the drain output, the electret microphone must provide DC The voltage can work, because it is equipped with a field effect tube.
Judging the polarity of electret microphones The method of detecting electret condenser microphones is: first check whether the pins are disconnected, and then detect the electret condenser microphones. The electret microphone is small in size, simple in structure, good in electroacoustic performance, low in price and widely used. The internal structure of the electret microphone is shown in the figure. It is composed of two parts: acoustic-electric conversion system and field effect tube. There are two ways to connect its circuit: source output and drain output. The source output has three lead wires, the drain D is connected to the positive power supply, the source S is grounded through a resistor, and then a capacitor is used for signal output; the drain output has two lead wires, the drain D is connected to the positive power supply through a resistor, and A capacitor is used for signal output, and the source S is directly grounded. Therefore, before using an electret microphone, it is necessary to determine its polarity.
A diode is connected between the gate and the source of the field effect tube, so the forward and reverse resistance characteristics of the diode can be used to distinguish the drain D and source S of the electret microphone.
Dial the multimeter to the R × 1kΩ position, the black test lead is connected to one pole, and the red test lead is connected to the other pole. Then swap the two test leads and compare the two measurement results. When the resistance is small, the black test lead is connected to the source and the red test lead is connected to the drain.
Structure and principle The electret microphone is composed of two parts: acoustic-electric conversion and impedance conversion.
The key element of the acoustic-electric conversion is the electret diaphragm. It is an extremely thin plastic film, on which one layer of pure gold film is evaporated. Then, after passing through the electret of the high-voltage electric field, opposite charges are resident on both sides. The steamed gold of the diaphragm faces outwards and communicates with the metal shell. The other side of the diaphragm is separated from the metal plate by a thin insulating collar. In this way, a capacitor is formed between the gold film and the metal plate. When the electret diaphragm encounters sound wave vibration, the electric field at both ends of the capacitor changes, resulting in an alternating voltage that changes with the sound wave. The capacitance between the electret diaphragm and the metal plate is relatively small, generally tens of pF. Therefore, its output impedance value is very high (Xc = 1/2 ~ tfc), about tens of megohms. Such a high impedance cannot be directly matched with the audio amplifier. Therefore, a junction field effect transistor is connected in the microphone to perform impedance transformation. The characteristics of the field effect tube are extremely high input impedance and low noise figure. The common field effect tube has three electrodes: source (S), gate (G) and drain (D). What is used here is a special field effect tube that combines a diode between the internal source and gate. The purpose of connecting the diode is to play a protective role when the field effect tube is impacted by a strong signal. The grid of the field effect tube is connected to the metal plate. In this way, there are three output lines of the electret microphone. That is, the source S is generally blue plastic wire and the drain D is generally red plastic wire and braided shielded wire connected to the metal shell.
Electret microphone sensitivity detection Electret microphones widely used in electrical appliances such as radio cassette recorders, telephones, etc., its sensitivity directly affects the performance of voice transmission and recording. The sensitivity of this type of microphone can be easily tested with a multimeter.
Set the multimeter to R × 100, the two test leads are respectively connected to the two electrodes of the microphone (be careful not to be wrongly connected to the ground electrode of the microphone). Even), observe the swing of the hands while blowing air. At the moment of blowing, the greater the amplitude of the needle swing, the higher the microphone sensitivity, and the better the speech delivery and recording effects. If the swing amplitude is not large (fine movement) or does not swing at all, it means that this microphone has poor performance and should not be used. The detection method for the three-pin electret condenser microphone is the same as above, except that the black meter rod is connected to the output pin 2 and the red meter rod is connected to the pin 3.
Electret microphone principle (picture)
The electret microphone is small in size, simple in structure, good in electroacoustic performance, low in price and widely used.
A certain permanent charge (Q) is injected during the production of the polymer polarizing film. Since there is no discharge circuit, the amount of this charge is constant. Under the action of sound waves, the polarizing film vibrates with the sound, so it is The distance also changes, that is, the capacitance between the lock polarizing film and the back pole changes with the sound wave.
We know that the formula for the charge on the capacitor is Q = C × V, and vice versa V = Q / C is also true. The total charge of the electret is constant. When the electrode plate retreats under the sound wave pressure, the capacitance decreases, and the voltage between the two poles of the capacitor will increase inversely. Conversely, when the capacitance increases, the voltage between the two poles Will decrease inversely. Finally, the voltage at both ends of the capacitor is taken out through the field effect with very high impedance and amplified at the same time, and we can get the voltage corresponding to the sound. Because the field effect tube is an active device, it needs a certain bias and current to work in the amplified state. Therefore, the electret microphone must be added with a DC bias to work.
Electret microphone optional electret microphone price is very low, after the damage to do replacement treatment, pay attention to the following points about electret microphone selection:
(1) The two- and three-pin electret microphones cannot be directly replaced, and in general, the circuit is not replaced.
(2) There is no model difference for this type of microphone. Microphones with the same pin count can be replaced, but there are differences in performance.
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